So I see. I forget that pulsed DC with limited return path (4.7M resistor) does not equal AC. This means that the circuit must be tied to an analog input on the AVR. Very interesting. It makes one have a bit of appreciation for the design work on these controllers.
Especially if there is a capacitor after it - that makes it a sort of poor man's integrator.
The location isn't the problem. It is the configuration.
Even if they got mounted in the worst possible location
1. Had they mounted the board upside down to the top of the box - No problem. Water would wick the wires and drip into the bottom of the case. The Switch is in the way so it can't be done now.
2. Had they used standoffs (I use 1-2 plastic grommets) - water capillary between the board and case would have been prevented... currently it is so close water is sucked in and can't evaporated due to capillary action (which reduces the surface area).
3. Conformal coating - had they conformally coated the board that would prevent most problems - both corrosion and shorts. I did this on mine using Plastic 70 (I think Dick used MG which should be better albeit both are Acrylic).
4. Drilling holes - that is what I was told they do today. At least it helps with evaporation - but it is very difficult to do in a density that would avoid the problems of capillary suction, etc.
The location itself is very good as it keeps the board cool. Installing it above or behind the boilers would have been awful.
My pet peeve is that they didn't mount it upside down in the box. 0 risks to board, 0 cost.